High molecular weight, water-soluble, cationic polymers are useful in a number of applications e.g. the flocculation of suspended solids, recovery of minerals from mining operations, papermaking, enhanced oil recovery, soil conditioning, etc. In many cases, the polymers are supplied to the user in the form of substantially dry polymer granules. The granules may be manufactured by the polymerization of water-soluble monomers in water to form a water-soluble polymer solution, followed by dehydration and grinding to form water-soluble polymer granules.
Another means for isolating the polymer from the polymer solution is to precipitate the polymer by mixing the polymer solution with an organic solvent e.g acetone or methanol that is a non-solvent for the polymer, then isolating the polymer by evaporation or filtration. However, in many cases, this method is inconvenient, expensive and dangerous because of the problem of handling large amounts of flammable organic solvent.
There have been a few reports of unusual precipitation behavior concerning particular cationic polymers. Certain novel cationic polyelectrolytes, termed ionene polymers, were reported (D. Casson and A. Rembaum, Macromolecules, Vol. 5, No. 1, 1972, pp. 75-81) to be insoluble in either 0.4 M potassium iodide or 0.4 M potassium thiocyanate. Poly(allylammonium chloride) was reported (T. Itaya et al., J. Polym. Sci., Pt. B: Polym. Phys., Vol. 32, pp. 171-177, 1994, and references 3, 5 and 6 therein, also Macromolecules, Vol 26, pp. 6021-6026, 1993) to precipitate in solutions containing the sodium salt of p-ethylbenzenesulfonate, or p-propylbenzenesulfonate or naphthalenesulfonate. Poly(4-vinyl pyridine) quaternized with butyl chloride and poly(allylammonium chloride) were reported (M. Satoh, E. Yoda, and J. Komiyama, Macromolecules, Vol. 24, pp. 1123-27, 1991) to precipitate in solutions of Nal and also in solutions containing the sodium salt of p-ethylbenzenesulfonate, respectively. It has also been reported (W-F. Lee and C-C. Tsai, J. Appl. Polym. Sci., Vol. 52, pp. 1447-1458, 1994) that poly(trimethyl acrylamido propyl ammonium iodide) did not dissolve in 0.5 M Na.sub.2 ClO.sub.4 or 0.5 M NaNO.sub.3.
Water-soluble polymers may also be supplied in the form of a water-in-oil emulsion or microemulsion, wherein the polymer solution droplets are isolated from each other by the continuous phase e.g. oil, of the emulsion or microemulsion. The polymer emulsions may be utilized directly in the desired application. Although this mode of supply is convenient and may avoid the need for dehydration, the oil may be expensive and is often flammable; in addition, the oil may also present a secondary pollution problem. Alternatively, the emulsion may be precipitated into an organic liquid that is a solvent for the water and oil, but a non-solvent for the polymer, followed by isolation and drying to recover the substantially dry polymer. However, these precipitation methods may be disadvantageous for the same reasons mentioned above.
A first water-soluble polymer may also be dispersed in the presence of a second water-soluble polymer to form aqueous polymer dispersions, as taught in U.S. Pat. Nos. 4,380,600 and 5,403,883. Since the two polymers do not dissolve each other, the first water-soluble polymer reportedly forms small globules which disperse in the solution of the second water-soluble polymer. Optionally, salt may be added to improve the flowability.
The polymers may also be precipitated in an aqueous salt solution. For instance, U.S. Pat. No. 3,336,270 discloses processes of polymerizing monomers to form water soluble polymers in aqueous solutions containing salt and tertiary butanol, wherein the polymer precipitates as it forms. U.S. Pat. Nos. 4,929,655 and 5,006,590; as well as EP 0 183 466 B1 and EP 0 630 909 A1, describe the polymerization of cationic water-soluble polymers in an aqueous solution which contains a multivalent anionic salt and a polymer dispersant, without the requirement of tertiary butanol. The aqueous solution of the multivalent anionic salt is a non-solvent for the polymer, so that the polymer precipitates as it forms. The polymer dispersant operates within the precipitation system by stabilizing the particles, but has no effect of depositing the polymer. The precipitation of the polymer depends on the functional group of the polymer and on the amount and identity of the salt. Polymer structural units which contain benzyl groups are more easily precipitated in salt solution than (meth)acrylamide structural units, which are themselves more easily precipitated than structural units such as (methacryloyloxyethyltrimethylammonium chloride), herein below DMAEM.MeCl, which do not contain benzyl groups. Typical cationic water-soluble polymers e.g. poly(methacryloyloxyethyltrimethylammonium chloride), hereinbelow poly(DMAEM.MeCl, tend to be soluble at room temperature in aqueous solutions of multivalent anionic salts such as those used in U.S. Pat. Nos. 4,929,655 and 5,006,590. As compared to the emulsions, the replacement of the oil by an aqueous salt solution was an advance in the art because the aqueous salt solution was not flammable and posed less of a secondary pollution problem. However, the presence of an aromatic benzyl group in the polymer may be a disadvantage from an economic and environmental perspective.
Polymers and monomers which contain hydrophobic groups e.g. benzyl may not themselves be hydrophobic because they may have a high degree of water solubility. However, polymers with hydrophobic groups tend to be more easily precipitated in salt solutions than polymers without hydrophobic groups. Although the benzyl group could be replaced by a hydrophobic alkyl group as in EP 0 525 751 A1, it would be clearly advantageous to eliminate the need for these hydrophobic groups entirely so as to avoid the expense and inconvenience of making the monomer which contains the hydrophobic group. Accordingly, there exists a need for compositions that may act as non-solvents for typical cationic water-soluble polymers presently used in applications such as water treating, mining, papermaking, oil recovery, etc., that are relatively non-flammable, inexpensive, and non-toxic.
The effect of salts on the solubility of various substances in aqueous solution is well discussed in the scientific literature. The "Hofmeister" series ranks anions according to their ability to increase or decrease the solubility of substances in water. Although positions in the ranking may vary slightly, depending on the substance, a generally accepted ranking of the anions is:
______________________________________ Salting-out SO.sub.4.sup.2- .about. HPO.sub.4.sup.2- &gt; F.sup.- &gt; Cl.sup.- &gt; Br.sup.- &gt; Salting-in (kosmotropic) I.sup.- .about. ClO.sub.4.sup.- &gt; SCN.sup.- (chaotropic) ______________________________________
It is well known that kosmotropic salts generally decrease the solubility of many substances in water. For instance, the Hofmeister ranking apparently guided the choice of salts for precipitating cationic water soluble polymers, containing hydrophobic groups, in U.S. Pat. Nos. 4,929,655 and 5,006,590, as well as EP 0 630 909 A1, EP 0 525 751 A1, and EP 0 657 478 A2, as evidenced by their use of strongly kosmotropic salts containing sulfate and phosphate anions. On the other hand, chaotropic salts generally increase the solubility of substances in water.
There are numerous means known to those skilled in the art for determining whether a particular salt is kosmotropic or chaotropic. Representative salts which contain anions such as sulfate, fluoride, phosphate, acetate, citrate, tartrate and hydrogenphosphate are kosmotropic. Representative salts which contain anions such as thiocyanate, perchlorate, chlorate, bromate, iodide, nitrate and bromide are chaotropic. The chloride anion is generally considered to be at about the middle of the Hofmeister ranking, being neither strongly chaotropic nor strongly kosmotropic. For our purposes, inorganic salts which contain the chloride anion are neither chaotropic nor kosmotropic.
Small amounts of sodium thiocyanate, for instance about 0.1% by weight, on total, have been reported to be useful as stabilizers for polymer dispersions as in EP 0 657 478 A2, where (NH.sub.4).sub.2 SO.sub.4 was used to deposit the polymer. Sodium thiocyanate and sodium iodide have been reported to be useful as stabilizers for hydroxylamine-containing water-soluble polymer systems, as in EP 0 514 649 A1. U.S. Pat. No. 3,234,163 teaches that small amounts of thiocyanate salts, preferably 0.1 to 1 percent, based on the weight of the polymer, are useful for stabilizing polyacrylamide solutions. The thiocyanate salts are claimed to stabilize the polyacrylamide by preventing or slowing molecular weight breakdown.
Many literature reports concerning the Hofmeister ranking of salts have included studies of their effects on low molecular weight substances which have relatively low water-solubility. However, the Hofmeister ranking has also been observed when the substrates were high molecular weight, water-soluble polymers. For instance, the effect of various salts on the solubility of synthetic, water-soluble polymers was explored by Shuji Saito, J. Polym. Sci.: Pt. A, Vol. 7, pp. 1789-1802 (1969). This author discussed the effect of various anions on polymer solubility and stated "This anionic order seems to be independent of the type of counter cations and is in line with Hofmeister's lyotropic series for anions." Similarly, in M. Leca, Polymer Bulletin, Vol. 16, pp. 537-543, 1986, the viscosity of polyacrylamide, as determined in 1N solutions of various salts, was found to increase in the order HPO.sub.4.sup.2- &lt;H.sub.2 O&lt;Br.sup.- &lt;NO.sub.3.sup.- &lt;I.sup.- =BrO.sub.3.sup.- &lt;ClO.sub.3.sup.- =SCN.sup.+. The viscosities were reported to be higher in more chaotropic salt solutions than in less chaotropic, or kosmotropic, salt solutions.
Certain anionic organic salts, such as hydrotropes and surfactants, also tend to increase the solubility of substances in water. The way that anionic organic salts act to increase the solubility of substances in water generally depends on the identity of the organic portion. Salts with smaller organic portions tend to function as hydrotropes, whereas salts with larger organic groups tend to function as surfactants.
The ability of surfactants to enhance the solubility of substances in water is well known. Compositions comprising sulphonated hydrocarbon surfactants and hydrophilic cationic polymers were disclosed in U.S. Pat. No. 5,130,358. For the purposes of this invention, surfactants are defined as surface active agents that have the ability to reduce the surface tension at an interface without requiring concentrations so large that the distinction between solute and solvent is blurred.
Surprisingly, and contrary to the teachings cited above, it has now been found that many typical water-soluble, cationic polymers such as poly(DMAEM.MeCl), which do not contain hydrophobic groups, can be precipitated in aqueous solution by the presence of a mixture of a chaotropic salt, or an anionic organic salt, and a kosmotropic salt. Therefore, in accordance with our invention, processes for precipitating water-soluble, cationic polymers by mixing water-soluble, cationic polymers with at least one chaotropic salt, or anionic organic salt, and at least one kosmotropic salt are provided. Compositions comprising water, at least one chaotropic salt, or anionic organic salt, at least one kosmotropic salt, and at least one precipitated cationic, water-soluble polymer are also embodied in the instant invention. Compositions in which the precipitated polymer is dispersed in the form of small droplets so as to produce a polymer dispersion are preferred. These polymer dispersions may be stabilized by a dispersant, which may be a different water-soluble polymer, and the precipitated polymer is preferably formed by polymerization of monomers in the salt solution, optionally in the presence of said dispersant. Processes for using these compositions to concentrate dispersions of suspended solids and to condition soil are also disclosed.